This invention refers to evaporators and more particularly, to evaporators for eliminating water contained in liquids and for cooling liquids by direct expansion of a refrigerant fluid.
The necessity of driving off superfluous water from products in order to render them more stable and concentrated is quite common in industry, particularly in the food industry. Typical examples are the production of evaporated milk, dehydrated coffee, fruit-juice concentrates, cane sugar, etc.
The most common type of evaporators for concentrating liquids comprises a heating unit consisting of a bank of tubes, usually vertical, arranged inside an enclosure or shell. Low pressure steam is admitted into the heating unit while the product to be concentrated or evaporated flows in contact with the tubes.
There are variations of this apparatus. In some of them the bank of tubes is arranged vertically; in others, the tubes are horizontal. There are also evaporators in which the product to be concentrated flows through the tubes while the steam flows externally thereof.
The efficiency of these evaporators is rather poor and they are usually arranged in series whereby the latent heat of the vapors is made to do duty several times. The shells are under vacuum and each suceeding shell has a higher vacuum and therefore, a lower boiling temperature than the one preceding it. The vapor from the boiling product passes over the heating unit of the second shell, where, due to the higher vacuum, the product boils at a lower temperature releasing vapor for boiling the material in the second shell and so on. The vapor from the final shell goes to a condenser.
Evaporators used in liquid cooling apparatus are part of a refrigeration circuit comprising a compressor, an expansion valve and a condenser. The evaporator, usually consisting of serpentine tubing or ducts, is either immersed into the liquid to be cooled (direct expansion system) or arranged in heat-exchange relationship with the fluid to be cooled.
Brine circulation is generally preferred to direct expansion so as to avoid danger from escaping ammonia or other refrigerant in case the tubing should leak.
Brine coolers are usually of the shell and tube or of the shell and coil type.
Thus, the evaporatros for concentrating products and the evaporators used in cooling liquids are essentially heat-exchangers operating basically on the same principle.
The quantity of heat transferred in this type of apparatus is governed by three main factors: (a) the extension and nature of the heat transfer surface exposed to both fluids, which depends on the number and length of the tubes; (b) the overall coefficient of heat transfer from one fluid through the intervening wall to the other fluid, which depends on the velocity of flow, and the shape and arrangement of the heating surface; and (c) the main temperature difference across the intervening wall from one fluid to the other.
For the fluid flowing through the tubes, the velocity is determined quite precisely by the flow rate and the number and diameter of the tubes. The velocity of the other fluid, which flows inside the shell externally of the tubes, also depends on the flow rate and the passage section defined among the tubes, but flow conditions may vary considerably from one area to another of the concentrator or evaporator.
Conventional evaporators are difficult to disassemble and assemble and have inaccessible surfaces which cannot be cleaned with simple methods or inspected visually in order to ensure that they adhere to adequate sanitary conditions. Therefore, these known evaporators are not well suited for use in applications where thorough and frequent cleaning of the internal parts of the apparatus is required, nor in processes which do not tolerate even minute amounts of contaminants.
For a more detailed discussion of the problems encountered in heat exchangers used in the food industry, reference is made to U.S. Pat. No. 4,402,361, sharing a common assignee with the instant application.
U.S. Pat. No. 4,402,361 refers to a heat exchanger comprising two or more frustoconical, coaxially superimposed jackets having a helical spacer therebetween which defines a helical fluid passage with the facing conical surfaces of the jackets. The helical spacer is freely and releasably mounted in the annular space between the jackets so that it may be removed for cleaning purposes or replaced by another one having a different geometrical configuration to vary the specifications of the heat exchanger.
The heat exchanger of this U.S. patent has proved to be very effective in treating food and pharmaceutical products.
It has now been found that the use of frusto-conical jackets with a helical spacer defining a helical channel therebetween is particularly effective in the evaporation of liquids where it provides unforeseen advantages and unexpected results.